Greenhouse gas balance over thaw‐freeze cycles in discontinuous zone permafrost

Abstract

Peat in the discontinuous permafrost zone contains a globally significant reservoir of carbon that has undergone multiple permafrost‐thaw cycles since the end of the mid‐Holocene (~3700 years before present). Periods of thaw increase C decomposition rates which leads to the release of CO2 and CH4 to the atmosphere creating potential climate feedback. To determine the magnitude and direction of such feedback, we measured CO2 and CH4 emissions and modeled C accumulation rates and radiative fluxes from measurements of two radioactive tracers with differing lifetimes to describe the C balance of the peatland over multiple permafrost‐thaw cycles since the initiation of permafrost at the site. At thaw features, the balance between increased primary production and higher CH4 emission stimulated by warmer temperatures and wetter conditions favors C sequestration and enhanced peat accumulation. Flux measurements suggest that frozen plateaus may intermittently (order of years to decades) act as CO2 sources depending on temperature and net ecosystem respiration rates, but modeling results suggest that—despite brief periods of net C loss to the atmosphere at the initiation of thaw—integrated over millennia, these sites have acted as net C sinks via peat accumulation. In greenhouse gas terms, the transition from frozen permafrost to thawed wetland is accompanied by increasing CO2 uptake that is partially offset by increasing CH4 emissions. In the short‐term (decadal time scale) the net effect of this transition is likely enhanced warming via increased radiative C emissions, while in the long‐term (centuries) net C deposition provides a negative feedback to climate warming.

Tropical peatland carbon storage linked to global latitudinal trends in peat recalcitrance

“…If the vegetation changes towards shrubs typical of pocosins and other lower-latitude peatlands (as is expected if warming leads to drier conditions 54 , 55 ), the increased release of plant-derived phenolics and other aromatics could prevent substantial carbon loss, possibly even inhibiting the decomposition of older Sphagnum and sedge peat 15 . If warmer wetter conditions favor an increased abundance of sedges (as is predicted for systems similar to our Stordalen site 56 , 57 ), substantial short-term decomposition and greenhouse gas release are much more likely, but this carbon loss may be balanced or exceeded by increased primary productivity 57 , 58 . In addition, the preferential decomposition of labile carbohydrates relative to more recalcitrant aromatics could then lead to long-term stability of catotelm peat 16 , 17 , 59 .…”

Soil Viruses Are Underexplored Players in Ecosystem Carbon Processing

“…These findings are also consistent with observations of distinct viral communities from desert, prairie, and rainforests ( 126 ) and from grasslands and arctic soils ( 45 ). In contrast, an emerging paradigm in the marine field is “seascape ecology” ( 127 ), where the majority of taxa are detected across broad geographical areas, as are marine viruses ( 7 , 26 ). This important difference in habitat specificity between soils and oceans may be due to the greater physical structuring of soil habitats.…”

“…High-latitude perennially frozen ground, i.e., permafrost, stores 30 to 50% of global soil carbon (C; ∼1,300 Pg) ( 2 , 3 ) and is thawing at a rate of ≥1 cm of depth yr −1 ( 4 , 5 ). Climate feedbacks from permafrost habitats are poorly constrained in global climate change models ( 1 , 6 ), due to the uncertainty of the magnitude and nature of carbon dioxide (CO 2 ) or methane (CH 4 ) release ( 7 ). A model ecosystem for studying the impacts of thaw in a high-C peatland setting is Stordalen Mire, in Arctic Sweden, which is at the southern edge of current permafrost extent ( 8 ).…”

Respiration of aged soil carbon during fall in permafrost peatlands enhanced by active layer deepening following wildfire but limited following thermokarst

“…The discrepancy between SR Deep and SR Aged occurred in all sites but was more pronounced in the thermokarst bog. It is well established that thermokarst bog development causes rapid accumulation of peat at the surface (Camill 1999, Turetsky et al 2000, Jones et al 2017, Wilson et al 2017. The large difference between SR Deep and SR Aged could thus be explained by the translocation and mineralization at depth of leachates/exudates with recently fixed C from the productive Sphagnum mosses and sedges.…”